The henna was mentionned to avoid, I am presuming, because it colors the hair. You can naturally also lighten your hair using honey and some cassia obovata mixed together. It takes time but the honey is a natural hydrogen peroxide. Just watch out to balance any hair treatments so that you keep your hair well moisturized.
Amla powder does add volume to the hair, and with any herbs yes they can become darkening.
Shikakai is a definete recommendation and any of these herbs can be blended together to make a special formula for yourself. It promotes hair growth and naturally cleanses the hair.
Another great product is ghassoul (rhassoul) clay from Morocco. Moroccan women are a true variety of african and arab people! The hair textures varies so much as well, that the natural products coming from there are amazing!! This clay is what is traditionally used in the hair to cleanse the hair. The natural ghassoul is great for that. Just apply in the shower under damp conditions and then wash out towards the end of your shower.
The herbal ghassoul is a great facial and body cleanser. With the herbs adding the perfect combination and exfoliation. This clay does NOT strip the natural oils from you. It is a HUGE hit!! I love that mud!!
I hope you find a good shop